Ultrapure water producing system

ABSTRACT

This disclosure relates to an ultrapure water producing system having a primary ultrapure water system including a pretreatment process into which raw water is fed, and a primary ultrapure water device connected to the outlet of the pretreatment process. A reservoir is connected to the outlet of the primary ultrapure water device. A secondary ultrapure water system includes a pump having an inlet connected to the reservoir, a polisher connected to the outlet of the pump, and first use point connected to the outlet of the polisher and to the reservoir. A branch line is connected to the outlet of the polisher and has a heating and deaerating part connected thereto, and a high-temperature second use point is connected to the outlet of the heating and deaerating part.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to an apparatus for producinghigh-temperature ultrapure water having a minimal amount of dissolvedoxygen and no increment of impurities such as total organic carbon(TOC), metallic ions, or the like, which is provided for improvement ina final cleansing procedure of components during semiconductormanufacturing process and also, to parts and materials of pipings ofsuch apparatus.

So-called ultrapure water is used in the manufacture of semiconductors.It is desired that such ultrapure water contain the least possibleamount of impurities including various ions, total organic carbon, fineparticles, or the like. Because the ultrapure water having the leastimpurities is highly active as a solvent, components materials of thesystem for handling it are primarily made of resin having lowleachability such as PVC (polyvinl chloride), PFA (Teflon™), PVDF(polyvinylidene fluoride), PEEK (polyetherether-ketone), and so on.Also, surface polished stainless steel is utilized when the ultrapurewater has to be heated up for the purpose of sterilization or the like.However, during the heating, some impurities are leached from thosematerials and increase up to about ten times the amount present at anambient temperature.

As very detailed work is required for processing integration ofsemiconductors, the spontaneous development of an oxide film (nativesilicon oxide) on the surface of a semiconductor is significant becauseit increases the contact resistance on the surface of a wafer and thusresults in a decline in the performance of integrated circuits. Such anoxide film is uniformly developed on the surface of each wafer by thedissolved oxygen in ultrapure water and causes defects in thearrangement of high-density circuit connection on the wafer. Thedissolved oxygen in the ultrapure water can be reduced to only 30 to 50ppb by a vacuum deaeration procedure. Hence, an improved method ofremoving a greater amount of dissolved oxygen from ultrapure water ismuch desired in this technical field.

At the final cleansing stage of a conventional process of producingsemiconductors, a heated inert gas such as argon gas or nitrogen gas isused for removal and drying out of the ultrapure water. To improve thecleansing procedure, it is preferable to maintain both the cleansingultrapure water and the wafer at a temperature of 80° to 90° C.

In the operation of a prior art ultrapure water producing system (seeFIG. 3), the raw water is first fed into a pretreatment process 34 of aprimary ultrapure water system 33 for removing suspended solids byflocculation, settling and filtering, and passed to a primary waterpurifying process 35 for removing dissolved salts and organic materialsby reverse osmosis and ion exchange. The resultant primary ultrapurewater from which a majority of the impurities have been removed is thentransferred to an ultrapure water reservoir 32 in a secondary ultrapurewater system 31.

The primary ultrapure water is then delivered by an ultrapure water pump36 from the ultrapure water reservoir 32 through a total organic carbondestruction device 37, a polisher 38, and an ultrafilter 39 for removingfurther impurities. A portion of the secondary ultrapure water at anambient temperature is supplied to a use point (point of use) 40 whilethe remaining portion is returned back to the ultrapure water reservoir32 through a valve 30. Also, another portion of the secondary ultrapurewater which is passed from the ultrafilter 39 flows into a branch line41 and is heated by a heater 43 mounted in an ultrapure water heatingdevice 42 up to a predetermined temperature and then fed into ahigh-temperature use point 44 for cleansing wafers.

The ultrapure water heating device 42 employs a material such astetrafluoride resin, synthetic quartz, or the like as the parts whichare in direct contact with the ultrapure water. The pipings for passingthe high-temperature ultrapure water are made of difluoride resin andthe like.

However, when the high-temperature ultrapure water is in direct contactwith the materials of the devices and pipings, then organic matters,fluorine, silica, and so forth are leached out thereby increasing theamount of impurities in the ultrapure water. Also, the ultrapure waterheating device 42 is capable of heating the secondary ultrapure waterbut not removing dissolved oxygen; therefore, it is inevitable that thedissolved oxygen will develop the native silicon oxide on the surface ofa semiconductor wafer during cleansing in the use point 44.

It is a primary object of the present invention to provide a system forproducing high-temperature ultrapure water for cleansing high densityintegrated circuit, which contains a minimal amount of dissolved oxygen,and to provide materials suitable for fabricating components and pipingswhich contact directly with the ultrapure water (referred to asmaterials hereinafter), more specifically, to provide a material forreducing the dissolved impurities in the ultrapure water to a negligibleamount.

SUMMARY OF THE INVENTION

Apparatus for heating ultrapure water according to the present inventionis adapted not only for heating up but also for deaeration at anelevated temperature and also, for reducing the dissolved oxygen in theultrapure water to less than about 2 ppb.

For accomplishing the above task, an improved arrangement is provided inwhich the secondary ultrapure water flows along a branch line extendingfrom the downstream side of a polisher disposed in a loop of thesecondary ultrapure water system, and then is directly fed into ortransferred via a pressurizing pump and/or a final filter such as anultrafilter to a high-temperature use point.

More particularly, a heating and deaeration apparatus for ultrapurewater according to the present invention includes a heating devicehaving a deaeration ability and provided in a branch line which extendsfrom a loop in the secondary ultrapure water system to ahigh-temperature use point.

The materials provided which contact directly with the high-temperatureultrapure water delivered from the heating and deaeration device isfabricated with austenite stainless steel which has beenelectrolytically polished and heated up in a high-temperature oxidizingatmosphere for developing a passive film thereon, whereby the leachingout of impurities such as metallic ions into the ultrapure water issuppressed.

More specifically, materials employed for parts and pipings whichcontact directly with the ultrapure water are made of stainless steelwhich is covered with a passive layer which has later been surfacecleaned with a weak acid.

The passivation is produced by buff finishing and electrolyticallypolishing the surfaces of the parts and pipings which contact directlywith the ultrapure water, and after complete removal of the electrolyticpolishing liquid, heating the exposed surfaces at 350° to 450° C. for 15to 30 minutes in an oxidizing atmosphere to form a tinted oxide film.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention is described as followswith reference to the figures of the attached drawings, wherein:

FIG. 1 is a flow diagram of an ultrapure water treatment system with aheating deaerating apparatus according to the present invention;

FIG. 2 is a cross sectional side view of a heating-deaeration device ofthe system; and

FIG. 3 is a flow diagram showing a conventional ultrapure waterproducing system.

DETAILED DESCRIPTION OF THE DRAWINGS

As shown in FIG. 1, raw water is first fed into a primary ultrapurewater system 1 including a pretreatment process 2 and a primaryultrapure water purifying device 3.

The raw water is clarified by flocculation, settling and filtering inthe pretreatment process 2, and then passed through a pH controller, areverse osmosis device, a vacuum deaeration device, an ion exchanger, amicron filter, and so forth provided in the primary ultrapure waterpurifying device 3, similar to the prior art system. The resultantprimary ultrapure water from which a majority of the impurities havebeen removed is then transferred to an ultrapure water reservoir 4 of asecondary ultrapure water system 5.

The primary ultrapure water is then delivered by an ultrapure water pump6 from the ultrapure water reservoir 4 to circulate in the secondaryultrapure water system 5 which includes a total organic carbondestruction device 7, a polisher 8, an ultrafilter 9 and a branch pipingto an end use point 11 all connected in a circulating loop 10 (polishingloop).

The ultrapure water from the reservoir 4 is pressurized by the pump 6and circulates in the secondary ultrapure water system 5, and thentreated by the total organic carbon destruction device 7 usingultraviolet rays of shorter wave length and deionized by the polisher 8for producing a secondary ultrapure water having a reduced amount ofimpurities.

For use at an ambient temperature, the secondary ultrapure water isfiltered by the ultrafilter 9 and directly fed from the loop 10 to theuse point 11. An outlet 29 for impurities may be taken from the filter9.

The remaining secondary ultrapure water which is not supplied to the usepoint 11 is returned back to the reservoir 4 through a valve 25.

In accordance with this invention, a branch line 12 is providedextending from between the polisher 8 and the ultrafilter 9 forsupplying the ultrapure water through a valve 25a to aheating-deaeration device 13 for producing high-temperature secondaryultrapure water.

As shown in more detail in FIG. 2, the device 13 includes a container26, and the secondary ultrapure water introduced into the container 26,which is connected to the branch line 12, is sprayed by spray valves 14towards the interior of the container 26 of the device 13 for heatingand primarily for deaeration.

The ultrapure water primarily deaerated by spraying is then fed into ascrubber 15 also mounted in the container 26. A portion of the secondaryultrapure water which flows from the branch line 12 to a steam generator16 (see FIG. 1), is evaporated by a heater 17 to a pure high-temperatureand high-pressure steam which is fed into the heating-deaeration device13 via a steam inlet 18 (see FIG. 2). The sprayed water falls into atray 27 and flows into a receptacle 28. The fed secondary ultrapurewater is turbulently mixed in the receptacle 28 with the pure steam fromsteam jets 15 for final deaeration to reduce the concentration ofdissolved oxygen down to about 2 ppb and then stored in the bottom 19 ofthe container 26.

This steaming process promotes the deaeration and, simultaneously,accelerates the removal of organic matters having lower boiling points.

A gas containing the deaerated oxygen from the water is passed through avent condenser 23 and discharged from the heating deaeration device 13through a vent 24.

At the same time, the vapor in the container 26 is condensed andreturned from the vent condenser 23 to the interior of the container andis mixed together with the secondary ultrapure water from the branchline 12.

Then, the heated and deaerated secondary ultrapure water is adjusted toa temperature between 80° and 90° C. by a heat exchanger 20 (FIG. 1) andtransferred via a high-temperature ultrafilter 21 to a high-temperatureuse point 22. An outlet 29a for impurities may be connected to thefilter 21.

The heating-deaerating device 13 may be any applicable type; preferably,a type which is able to heat the ultrapure water to higher than asaturating temperature (e.g. 105° to 130° C.) at atmospheric pressure.It should also include a tray type device for increasing the contactarea between ultrapure water and materials, or a device incorporating asteam or electric heater may be employed.

If the heating deaeration is carried out at a temperature of below 100°C., a negative pressure generator such as an ejector or a vacuum pup isrequired for maintaining the interior of the heating-deaeration deviceat a lower pressure than the atmospheric pressure. In addition, apressurizing pump needs to be provided at the output side of theheating-deaeration device for supplying the water.

Further, the secondary ultrapure water to be heated and deaerated may bebranched to the device 13 from the loop 10 (FIG. 1) on the downstream ofthe ultrafilter 9 before the use point 11, rather than before the filter9. In this case, the high-temperature ultrafilter 21 is not needed.

The parts of the heating-deaeration device 13, the pressurizing pump forsupplying the water, the heat exchanger 20, the high-temperatureultrafilter 21 and at least the pipings on the downstream sides of thesedevices, which contact directly with the high-temperature ultrapurewater, are made of austenite stainless steel. The surfaces of the steelthat contact the high-temperature ultrapure water are electrolyticallypolished and covered with a passive film which is formed by heating upthe part in a high-temperature oxidizing atmosphere. In this manner,there is avoided a decrease of the purity of the heated-deaeratedultrapure water which would otherwise be brought by the leaching out ofimpurities such as metallic ions from the materials of the parts andpipings.

The passivation is carried out according to the following steps: Theinner surfaces of the heating device 16 and inlet/outlet pipings thereofare made of SUS316 stainless steel and are buff finished using #600abrasive agent; they are then electrolytically polished to produce amirror surface, cleaned thoroughly with pure water, formed with a tintedoxide film (amorphous thin film of metal oxide) by being heated up to350° to 450° C. for 15 to 30 minutes, and cleaned with a 300 ppmlactic-acid water solution.

It should be noted that when the heating temperature is less than 350°C., the formation of a passive film becomes imperfect. If over 450° C.,a chrome carbide precipitates in the film which makes the film fragile,thereby accelerating the leaching of metallic ions.

Since, during heating, the Fe component in stainless steel is initiallyoxidized and forms a primary film, when the primary film abundant withthe Fe component is removed by washing with a weak acid, the leaching ofFe-ions into the ultrapure water can be minimized.

The passivated stainless steel not only has less leachability ofimpurities such as metallic ions, but it also has a smooth surface whichis sufficient to avoid the sticking of germs. Further, it makes minimumaffect of the ozone which is added to the ultrapure water forsterilization and destruction of organic carbon. In addition, it will bepreferably regarded also as an optimum material for the fabrication ofparts and pipings of the apparatus used at an ambient temperature.

EXAMPLE 1

The characteristics (value) of the ultrapure water supplied at thehigh-temperature use point 22 of the ultrapure water apparatus (FIG. 1)according to the present invention and also, at the high-temperature usepoint 44 of the conventional ultrapure water apparatus (FIG. 3) areshown in following Table 1.

                  TABLE 1                                                         ______________________________________                                                           Present  Conventional                                                         Invention                                                                              Technology                                        Item               (FIG. 1) (FIG. 3)                                          ______________________________________                                        Specific Resistance (MΩcm)                                                                 18.2     18.2                                              Total Organic Carbon (μg/l)                                                                   1.0      8.0                                               Bacteria (Cells Per 100 cc)                                                                      .0       .0                                                Silica (μg/l)   <1       <1                                                Dissolved Oxygen (μg/l)                                                                       <2       90                                                ______________________________________                                    

EXAMPLE 2

The results of a leaching experiment of unfavorable substances are shownin the following Table 2 in the comparison between a stainless steelhaving passivated contact surface thereof with ultrapure water and astainless steel having polished and smoothed contact surface thereof.

The experiment was carried out using a heating device 16 (inner surfacearea 0.291 m² and water content 7.4 liters) having passivated contactsurface and, also, the same (having polished inner surface of SUS316stainless steel buff finished using #600 abrasive agent) in which theultrapure water remained at 80° C. and was kept sealed for five days.

The leached out substances were analyzed using the methods of framelessatomic-absorption spectroscopy, ICP-MS, ion-exchange chromatography, andwet oxidation TOC meter, so that error in the measurements was minimizedin each leached out substances.

                  TABLE 2                                                         ______________________________________                                        Present Invention   Conventional Technology                                           Before  After         Before                                                                              After                                             Exper-  Exper-  Differ-                                                                             Exper-                                                                              Exper- Differ-                            Item    iment   iment   ence  iment iment  ence                               ______________________________________                                        TOC*    59.0    60.0    +1.0  54.0  72.0   +18.0                              Fe      0.84    0.67    -0.17 1.4   18.0   +16.6                              Ni      0.23    1.1     +0.87 0.20  31.0   +30.6                              Mn      <0.05   1.3     +1.25 <0.05 9.5    +9.45                              Cr      0.10    <0.05   -0.05 0.10  <0.05  -0.05                              Na      0.35    0.38    +0.03 0.11  0.92   +0.81                              K       0.03    0.15    +0.12 0.03  0.25   +0.22                              Ca      0.63    1.10    +0.47 0.30  1.20   +0.90                              Mg                            <0.05 0.31   +0.26                              Zn      0.10    0.20    +0.10 0.05  0.92   +0.87                              Cl      0.07    0.12    +0.05 0.07  0.40   +0.30                              SO.sub.4                      <1.0  5.0    +4.0                               NH.sub.4                                                                              0.10    <0.05   -0.05 0.07  4.9    +4.83                              SiO.sub.2 (Ion)                                                                       <1.0    3.0     +2.0  <1.0  11.0   +10.0                              Cu      0.07    0.40    +0.33                                                 ______________________________________                                         *Total Organic Carbon                                                    

Above Table 2 gives numerals in the unit of μg/l, in which minus valuesassociated with the difference between before and after the experimentresult from error in the analysis.

As will be apparent from the results of the experiment, the leaching outof substance with apparatus according to the present invention isremarkably reduced, particularly in Fe, Mn, and NH₄, considering theanalytic error.

EXAMPLE 3

The same experiment as of Example 2 was carried out using conventionalmaterials of tetrafluoride resin (PFA), difluoride resin (PVDF),polyetherether-ketone (PEEK), and the material of the present invention.They were then compared with respect to the leaching out of impurities.

The results indicated in the unit of mg/m² are shown in the followingTable 3.

                  TABLE 3                                                         ______________________________________                                        Item     Present Invention                                                                          PFA      PVDF  PEEK                                     ______________________________________                                        TOC*     0.025        4.8      17    3.5                                      Na        0.0007                     0.15                                     K        0.003                       0.07                                     Ca       0.012                        0.113                                   Cl        0.0013                     0.11                                     ______________________________________                                         *Total Organic Carbon                                                    

As will be apparent from the above-mentioned results, the leaching outof organic impurities from the conventional materials PFA, PVDF andPEEK, according to the present invention is 1/190, 1/670 and 1/140,respectively. The leaching of Na, K, Ca and Cl from PEEK is as low as1/210, 1/230, 1/10 and 1/80, respectively. All the results clearlyindicate the decrease in leaching of unfavorable impurities from thematerials.

Accordingly, the ultrapure water producing apparatus of the presentinvention is capable of producing the ultrapure water which contains aless amount of dissolved oxygen, and thus is suitable for use ascleansing water in the semiconductor chip manufacturing process, ascompared with the conventional technology which depends on a procedureof vacuum deaeration.

Also, the material according to the present invention has less leachableamounts of unfavorable impurities to be leached out from its contactarea exposed to the high-temperature ultrapure water producingapparatus.

The present invention offers improvement in the operation efficiency ofcleansing by providing a consistent supply of high-temperature ultrapurewater which is improved in cleansing capability and contains fewerimpurities.

What is claimed is:
 1. An ultrapure water producing systemcomprising:(a) a primary ultrapure water producing means including afiltering means for receiving raw water, and a primary ultrapure waterpurifying means following said filtering means; (b) a reservoirfollowing said primary ultrapure water purifying means; (c) a secondaryultrapure water producing loop comprising said reservoir, a pumpfollowing said reservoir, a polisher following said pump, and a firstuse point connected at a point between said polisher and said reservoir;(d) a branch line extending from said polisher and comprising aheating-deaeration means following said polisher, and a high-temperaturesecond use point following said heating-deaeration means, and (e) saidbranch line having an inner surface of an austenite stainless steelhaving an electrolytically polished surface and a tinted oxide filmthereon.
 2. A system according to claim 1, wherein said branch line isconnected at a point between said polisher and said ultrafilter, andsaidsecondary ultrapure water producing loop further comprises: (f) a totalorganic carbon destruction device connected at a point between said pumpand said polisher; and (g) an ultrafilter connected at a point betweensaid polisher and said first use point.
 3. A system according to claim1, wherein said secondary ultrapure water producing loop producesultrapure water, and includes means for producing pure steam by heatinga portion of said ultrapure water, and wherein said heating-deaerationmeans comprises means for contacting said ultrapure water with said puresteam.